Great improvements in optical coating technology have been achieved through the introduction of multiple layer films. In fabricating multiple layer films, two different materials are typically used--one with a relatively high index of refraction and the other with a relatively low index of refraction. The two materials are alternately deposited in a controlled sequence of thicknesses to obtain the desired optical characteristics for the film. The deposition process is typically controlled by monitoring the thickness of each layer as it is deposited and by terminating the deposition when the layer reaches the correct thickness. This approach provides the flexibility to design a wide range of multiple layer interference coatings for various transmission and reflection spectra. The result is the addition of complex spectral filter structures to many new optical devices. Antireflection coatings, laser dielectric mirrors, television camera edge filters, optical bandpass filters, and band-rejection filters are some of the examples of useful devices employing multilayer thin film interference coatings.
Some advanced applications of optical technology, however, have performance requirements that exceed the capabilities of multiple layer thin films. New optical design procedures have been developed for these advanced applications to predict the continuous refractive index profile required for any desired transmission or reflection spectrum, including multiple reflectance bands. These design techniques employ gradient index layers, in which the index of refraction varies continuously as a function of depth into the layer. Gradient index optical coatings have advantages over conventional technologies, including flexibility in filter design and increased stability in adverse environments.
One type of gradient index structure is the rugate filter, the simplest manifestation of which has a periodic refractive index that varies sinusoidally with respect to optical thickness. A rugate filter is a gradient index analog of a quarterwave stack reflector. Compared to a quaterwave stack, however, a rugate filter has greatly suppressed high-frequency reflection harmonics. A rugate filter can provide high reflectivity within a narrow bandwidth simply by increasing the number of periods in the filter.
In the past, practical realizations of the rugate and other gradient index structures were inhibited by the limitations of thin film fabrication technology. However, improved methods of monitoring and controlling the deposition of optical thin films, including gradient index films, have greatly advanced the technology of rugate filters having continuous refractive index profiles. Background information regarding this technology can be found in the following patents, which are hereby incorporated by reference: U.S. Pat. No. 4,707,611 issued to Southwell for "Incremental Monitoring of Thin Films"; U.S. Pat. No. 4,778,251 issued to Hall et al. for "Thickness Error Compensation for Digital Gradient-Index Optical Coatings"; U.S. Pat. No. 4,934,788 issued to Southwell for "Deposition of Gradient Index Coatings using Coevaporation with Rate Control"; U.S. Pat. No. 4,952,025 issued to Gunning, III for "Rugate Filter Incorporating Parallel and Series Addition"; and U.S. Pat. No. 5,000,575 issued to Southwell et al. for "Method of Fabricating Gradient Index Optical Films."
A remarkable feature of gradient index optical filters is that sinusoidal index variations may be superimposed (i.e., added) to produce multiple rejection bands (also referred to as lines, reflectance bands, or stop bands) with a filter having the same thickness as required for a single rejection band. However, it has been thought that the number of stop bands or lines producible in a rugate filter is limited by the range of the refractive index material available for the deposition process used to fabricate the filter. As more sinusoidal variations are superimposed, the resulting variation profile can greatly exceed the refractive index range producible by available materials. Thus, there is a need for a method of fabricating gradient index optical films having multiple stop bands using currently available deposition materials.